Chromium plating



United States Patent 3,334,033 CHROMIUM PLA'HNG Edward A. Romanowski,Troy, and Henry Brown, Huntington Woods, Mich., assignors to The UdyliteCorporation, Warren, Mich., a corporation of Delaware No Drawing. FiledAug. 6, 1965, Ser. No. 477.937 22 Claims. (Cl. 20451) This applicationis a continuation-in-part of our c0- pendin-g application, Ser. No.395,892, now abandoned, filed Sept. 11, 19 64.

This invention relates to the electrodeposition of chromium from aqueousacidic exavalent chromium solutions, and especially to the use ofsaturation concentrations of certain. rare earth salts in solutionscontaining the sulfate ion to make possible improved chromium plate fromoperationally simplified baths. The invention also relates to the use offluorocarbon acids in these improved chromic acid baths to provide bathswhich give further improved covering power in recessed areas (lowcurrent density areas). Not only is the chromium covering powerimproved, but no strong discoloration or iridescent films are formedwhere the chromium plate leaves off in the very low current densityareas.

In its broad aspects, the invention contemplates the use of saturationconcentrations of certain rare earth salts in chromium plating bathscontaining from about 100 to about 500 grams per liter of chromic acidwith a chromic acid to sulfate ion ratio of from about 75 to 1 to about300 to 1. With a highly preferred embodiment, the bath containssaturation concentrations of strontium sulfate, and still furtherimprovements can be obtained by including certain fluorocarbon acids inthe bath.

The salts useful in the plating baths of this invention arefluorine-containing salts of certain rare earth metals includingsalts ofpraseodymiurn, neodymium, lanthanum, samarium and gadolinium. As usedherein, the term rare earth is also used to include yttrium. Though thislatter element is a Group IIIb element, it has properties similar to andis found associated with rare earths in nature. The useful salts are thefluorides or complex fluorides of these metals as hereinafter defined.

The chief commercial source of the rare earth metals is the naturallyoccurring phosphate ore known as monazite. In addition to the rare earthmetals, this ore also contains among others, thorium and yttrium. Thenomenclature and separation techniques involved in the refining of themonazite ore are defined in a publication of the Lindsay Division of TheAmerican Potash and Chemical Corporation entitled, Thorium, Rare Earthand Yttrium Chemicals.

It has now been found that fluorides and/ or complex fluorides ofcertain rare earth mixtures derived from the monazite ore areparticularly useful for the purpose of this invention. These mixtures asdefined in the above publication are termed, didymium salt, neodymiumsalt and lanthanum salt. Didymium refers to the mixture of rare earthsobtained after removal of cerium and thorium from the natural mixture ofrare earths found in the monazite ore. The approximate composition ofthe mixture, in terms of the oxides, is 40-45% La O 812'% Freon, Nd203,$111 03, Gd203, and trace amounts of other rare earth metal oxides.

The didymium mixture may be further processed and a lanthanum portionremoved therefrom. This fraction, mainly lanthanum oxide, may also beused to provide the desired fluoride salts. Similarly, the remainder ofthe didymium mixture from which the lanthanum has been removed can beconverted to the fluorides or complex fluorides and included in thebaths of this invention. This mixture, termed as neodymium salts by the3,334,033 Patented Aug. 1, 1967 Lindsay publication has the followingcomposition: 65- 70% Nd O 1216% Pr O 1013% Sm O 35% Gd O and traceamounts of other rare earth oxides.

The neodymium salts can be further refined to provide a variety ofmixtures of the remaining rare earth metals or the individual rare earthmetals can be obtained therefrom. While the fluorides of any of thesemixtures or the fluorides of the remaining individual rare earths can beused in the baths of this invention, the cost of further separationbecomes increasingly higher. Thus the preferred salts of this inventionare the fluorine containing salts of didymium, neodymium and lanthanumas defined above. These mixtures as the oxide, the carbonate, chloride,fluoride, etc., are available commercially.

The trivalent rare earth ions and especially the didymium ions form afluoride (a mixture of LaF NdF PrF SmF GdF3) which was found to have thedesired low solubility in dilute or concentrated chromic .acid solutionsat temperatures in the range of 2080 C.

Tetravalent ceric fluoride (or trivalent cerous fluoride which isoxidized to the tetravalent form at the lead anode) is too soluble inchormic acid solutions and when used in saturation concentrationscontributes excessive fluoride ions resulting in a diminution incovering power of the chromium plate.

It was found that saturation concentrations of mixtures such as didymiumfluoride, neodymium fluoride and lanthanum fluoride as well as theindividual rare earth metal fluorides enumerated above gave excellentresults by providing ideal controlled concentrations of fluoride orcomplex fluoride anions in chromium plating baths having a chromic acidto sulfate ion ratio of from about to 1 to about 300 to 1. With apreferred bath, the sulfate ion is present in a saturation concentrationas strontium sulfate and the chromic acid to sulfate ion ratio is fromabout to 1 to about 250 to 1. With such solutions, there is no need ofusing suppressing salts for the fluoride or complex fluoride as requiredby prior art teachings.

As previously pointed out, the invention contemplates the use of certainrare earth metal fluorides, and more particularly didymium, lanthanum orneodymium fluorides or complex fluorides of these rare earths such asfluosilicates, fiuoborates, fluoaluminates, fluotitanates,fluozirconates, etc., in chromic acid plating baths. These fluorides, orcomplex fluorides may be added as such to the plating bath.Alternatively, they may be formed in situ by adding salts of thesemetals, preferably the carbonate or hydrated oxides (hydroxides) tochromium plating baths containing a more soluble fluoride or complexfluoride. As used hereinafter, unless otherwise stated, the termfluoride shall be understood to also include complex fluorides asdefined above.

In order to electrodeposit chromium from acidic hexavalent chromiumsolutions, it is well known that small amounts of certain anions such assulfate or fluoride ions termed catalyst ions must be present. Actuallysome sulfate ion is required in the bath before fluoride ions cooperateto act as a catalyst. The use of mixed catalyst anions, such ascombinations of sulfate ions with fluoride ions, or with boric acid hasbeen previously suggested in chromium plating baths. In this connection,reference is made to US. Patents 1,844,751, 1,864,013, 1,864,014,1,952,793, 2,042,611, 2,063,197, 2,640,021, 2,640,022 and 2,952,590. Inparticular, reference is made to Lukens US. 2,042,611, Passal US.2,640,021 and Stareck US.

2,640,022 and 2,952,590 for the development of the selfcontaining up toabout 200 grams per liter of CrO Passal improved on this by addingsparingly soluble potassium silicofluoride (fluosilicate) to the bathcontaining saturation concentrations of strontium sulfate. Stareck madefurther improvements by additionally using mixed suppressing agents suchas strontium chromate and potassium dichromate to better control theconcentrations of the sulfate and silicofluoride ions respectively bycommon ion effects. Stareck also employed fluoaluminate, fluotitamateand fluozirconate ions instead of fluosilicate to use with controlledsulfate ion concentrations.

The standard or conventional chromium bath employing only the sulfateanion as catalyst and used for plating on nickel, iron, yellow brass orcopper has utilized a chromic acid anhydride (CrO to sulfate ion ratioof about 100 to 1. Thus, in a 200 grams per liter CrO bath, 2 grams perliter of sulfate ion would be used. If one uses instead a CrO to $0.;ratio of 200 to 1 or higher and plates on top of nickel, copper, yellowbrass or steel, instead of chromium plate, only an iridescentnon-metallic chromium chromate (rainbow) film is obtained when deadelectrical entry is used.

A specific example is a bath containing about 340 grams/ liter ofchromic acid and saturated with strontium sulfate at about 50 C. Ifchromium plating is attempted on top of a freshly plated bright nickelsurface using a dead entry into such a bath, no chromium plating isobtained, but only a non-metallic iridescent film of a basic chromicchromate results. The term dead entry refers to the technique whereinthe plating current is turned on only after the nickel plated work isimmersed in the bath.

It has now been found that saturation concentrations of certain rareearth fluorides as defined above are unusual in cooperating with thesulfate ion in the chromic acid baths to produce bright chromium plateof exceptional covering power on nickel, brass, copper and steel. Usingsuch a bath, it is possible to deposit bright chromium plate over a verywide cathode current density range. Outstanding results are obtainedusing saturation concentrations of the rare earth fluorides incombination with saturation concentrations of strontium sulfate.

The rare earth fluoride is used in saturation concentrations andpreferably an excess should be present undissolved in the bath. From 5to grams/liter are more than sufficient to provide this excess.Saturation concentrations of the rare earth fluorides in the chromicacid baths do not cause any harmful effect in conjunction withsaturation concentrations of strontium sulfate, such as greatlydiminishing the covering and throwing power of the chromium deposit andproducing dull white areas in the high current density chromium plate.In this very important respect, the saturation concentrations of rareearth fluorides such as didymium or neodymium fluoride act quitedifferently than saturation concentrations of other inorganic fluorideswhich were tested, such as calcium fluoride, strontium fluoride, cericor cerous fluoride, potassium or sodium fluosilicate (silicofluoride),etc. These latter fluorides and silicofluorides when used in saturationconcentrations in chromic acid solutions with saturation concentrationsof strontium sulfate diminish the covering and throwing power of thechromium plate and cause white areas in the high current densitychromium plate. It was for this reason that with prior art baths,suppressing agents such as potassium dichromate were required inconjunction with saturation concentrations of fluorides such aspotassium silicofluoride. In addition to the disadvantages of handlinganother material and introducing one more variable into the bath, thepresence of high concentrations of potassium ions tend to salt out theimportant and valuable anti-misting agent, perfluoro n-octyl sulfonicacid, which is often present in chromic acid baths. With the baths ofthis invention, the saltingout problem is totally eliminated in that theneed for agents to suppress the fluoride ion is totally eliminated.

The covering power of the chromium plate is maximum when it is platedover a bright nickel deposit obtained from a freshly prepared bath or abath that has been treated with activated carbon. The chromium plateapplied to such bright nickel has excellent covering power and is theleast susceptible to whitish streaks. In contrast, when the nickel bathhas been heavily used and has not been treated with activated carbon forlong periods of time, the covering power of the chromium plate isdefinitely diminished. The bright nickel deposits from the latter nickelplating baths are more passive. This condition often requires a higherconcentration of sulfate ion or fluoride ion in the chromium platingbath than would otherwise be necessary to obtain a good bright chromiumplate free of stained low current density areas. These higherconcentrations of catalyst ions, however, reduce the covering power ofthe chromium plate. Nevertheless, it is often necessary despitereduction of the covering power to use higher catalyst concentrations toeliminate or minimize staining in the low current density areas. Forthis purpose, a very serious problem is presented in using relativelysoluble fluorides such as ceric fluoride, ceric fluoborate, calciumfluoride, etc., to supply extra fluoride ions to the bath to reduce thepassivity of the nickel plate. Not only is it difficult to analyze forthe proper amount of fluoride in the bath, but if an excess isinadvertently added, it is diflicult to reduce the concentration to theproper level without resorting to dilution of the bath.

With baths of the present invention, it is not possible to add an excessof fluoride ion because of the limited solubility of the particular rareearth fluorides of this invention. It is thus possible to keep aconstant concentration of fluoride ion in the chromium plating bath, andto increase the concentration of sulfate ion to take care of excessivepassivity of the nickel plate. The sulfate ion concentration can easilybe determined, and if an excess is accidentally used, it is easilyreduced by addition of strontium chromate or carbonate. In general,saturation concentrations of strontium sulfate provide the properconcentration of sulfate ion to cooperate with fluoride ions provided bythe saturation concentration of the particular rare earth fluoridesdescribed above. This is especially true for the lower range of bathconcentrations of chromic acid.

The problem of chromium plating on a relatively passive bright nickelcan be further minimized by including in the chromium plating bath fromabout 0.5 to 5 grams per liter of an aliphatic or cycloaliphaticfluorocarbon acid such as fluorocarbon sulfonic acid or a fluorocarbonphosponic acid. Examples of these activators include perfluorocyclohexylsulfonic acid, perfluoro para methyl cyclohexyl sulfonic acid, perfluoropara ethyl cyclohexyl sulfonic acid, perfluoro succinic acid, perfluoromethyl sulfonic acid or a fluoroalkyl phosphonic acid such as H(CF CF),,PO(OH) where m=1 to 3 inclusive. These activators not only areeffective with the more passive nickel surfaces to minimize gray in thehigh current density areas, but also act to increase the chromiumcoverage.

The acidic hexavalent chromium plating baths may be made up fromstraight chromic acid anhydride or chromic acid, or from mixtures withdichromates, chromates, and polychromates. It is generally preferred touse straight chromic acid or chromic acid anhydride. The presence ofcations such as Na, K, -Li, Mg and Ca are best kept low inconcentration, especially K and Na.

Below are listed several examples of the chromic acid baths of thisinvention. Where strontium sulfate is employed at saturationconcentrations in the chromium plating baths, strontium chromate,bichromate or carbonate can also be added to partially suppress thesulfate ion concentration. This is often desirable in the low metalbaths such as those employing about to about 200 grams/ liter of chromicacid anhydride.

5 EXAMPLE I 150 to 340 grams/ liter of chromic acid anhydride (CRO'Saturation concentrations of SrSO (excess present), a ratio of chromicacid to sulfate of about 120 to 1 to about 250 to 1-SrCrO or srCr O atto 10 grams/ liter Saturation concentrations of didymium fluoride (2 to10 grams/liter, an excess is present), or the fluosilicateTemperature--115130 F. (46-55 C.)

EXAMPLE II 150 to 400 grams/liter of CrO Saturation concentrations ofSrSO, (excess present), a ratio of chromic acid to sulfate ion of about120 to 1 to about 250 to 1 Saturation concentrations of didymiumfluoride or fluosilicate or fluoborate (2 to 6 grams/liter, an excess ispresent) 1 to grams/liter of perfluoro para ethyl cyclohexyl sulfonicacid Temperature-105140 F.

EXAMPLE III EXAMPLE IV 200 to 400 grams/liter CrO Saturationconcentrations of neodymium fluoride or flu'osilicate (2 to grams/liter,an excess is present) Saturation concentrations of strontium sulfate,120 to 1 to about 300 to 1 of chromic acid to sulfate ion 1 to 5grams/liter of perfluoro para ethyl cyclohexyl sulfonic acid 0 to 6grams/liter of CF SO H and/or where n=an integer from 1 to 3 inclusiveTemperature105-140 F.

EXAMPLE V 200 to 400 grams/liter chromic acid anhydride 0.5 to 4 grams/liter (saturation concentration) of didymium fluoride 4 to 6 grams/literstrontium sulfate (saturation concentration) 4 to 10 grams/ liter ofstrontium chromate 0.5 to 5 grams/liter of perfluoro p-ethyl cyclohexylsulfonic acid 0 to 5 grams/liter of H(CF CF PO (OH) where 11:1

to 3 inclusive Temperaturel-O0-140 F.

EXAMPLE VI 100 to 500 grams/liter chromic acid anhydride 1 to 5grams/liter (saturation concentration of didymium fluoride) 0.3 to 4grams/liter sulfuric acid (an amount sufficient to have a CrO /SO ratioof about 120-300 to 1) Temperature100-l40 F.

Examples VII-IX which follow are included to show by direct comparisonthe superiority of the plating baths of this invention as compared witha bath utilizing a relatively soluble metal fluoride. With theseexamples, three 3" x 4" brass panels were first plated in an identicalmanner using a standard bright nickel bath. One panel was then plated ina chromium plating bath of this invention utilizing didymium fluoride(Example VII). The second panel was plated in the identical chrome bathof Example VII, but wherein didymium fluoride was replaced with ceriumtetrafluoride (Example VIII). The third panel Was plated in a bath as inExample VIII, but wherein the 80., concentration was decreased toprovide a CrO /SO ratio of 30 0/ 1 (Example IX). The chromium platingwas carried out in a standard Hull Cell. This cell is widely used by theplating industry to investigate and compare the throwing (or covering)power of various plating solutions. The test panel is placed at anoblique angle to the anode and plated for a period of 5 minutes. Thepercentage of the panel plated provides a direct measurement of thecovering power of the plating bath. These tests are especially useful incomparing the covering power of several different baths of the samemetal.

EXAMPLE VII 300 grams/ liter CrO 10 grams/liter didymium fluoride(greater than the saturation concentration) 4 grams/liter strontiumsulfate (greater than the saturation concentration) Temperaturel20 F.

After 5 minutes of plating, approximately of the panel was plated with aclear, brilliant plate.

EXAMPLE VIII EXAMPLE IX 300 grams/ liter CrO 10 grams/ liter CeF 1 gram/liter H 80 Temperaturel20 F.

With this example, the CrO /SO ratio was increased to about 300 to 1 inan attempt to improve the covering power of the bath of Example VIII.While approximately 70% of the panel was plated after 5 minutes, theplate was a poor quality showing whitish cloudy areas throughout. Such aplate is definitely not acceptable for a commercial application onbright nickel.

EXAMPLE X to 300 grams/liter CrO 0.5 gram/liter up to saturationconcentration of didymium fluosilicate, didymium fluoride or didymiumfluoborate Strontium sulfate or H 80 in amounts to give CrO S0 ratios offrom about 75 to 1 to about 300 to 1.

Another advantage of the baths of this invention is that there isnegligible etching of unplated steel. For example, a steel plateimmersed for a week in these chromium baths containing the slightlysoluble didymium or neodymium fluorides or fluosilicates present insaturation concentrations is affected to a far less degree compared toother chromium baths containing the more soluble fluorides and/orfluosilicates. This is very important in hard chrome (thick chromium)plating applications.

The particular rare earth fluoride or fluorides can be added directly tothe bath as such, which is the preferred method, or formed from thecarbonate or the hydrated rare earth oxide. Instead of using theparticular rare earth hydroxide or carbonate, the chromate, bichromate,or the sulfate of the particular rare earths used in this invention canbe formed into the fluorides, fluosilicates, fluoaluminates,fluoborates, etc., by reaction with hydrofluoric acid, fluosilici-cacid, etc. If excess sulfate ions are present they can be reduced inconcentration by adding strontium chromate or carbonate or hydroxide.

We claim:

1. A bath for the electrodeposition of chromium plate comprising about100 to 500 grams per liter of chromic acid, sulfate ion in aconcentration sufficient to have a chromic acid to sulfate ion ratio offrom about 75 to 1 to 300 to l, and saturation concentrations offluorinecontaining salt of a rare earth metal selected from the groupconsisting of neodymium, praseodymium, lanthanum, gadolinium, sarnarium,yttrium, and mixtures of said salts.

2. A bath in accordance with claim 1 containing saturationconcentrations of strontium sulfate.

3. A bath in accordance with claim 1 wherein said salt is a didymiumsalt.

4. A bath in accordance with claim 1 wherein said salt is didymiumtrifiuoride.

5. A bath in accordance with claim 1 wherein said salt is didymiumfiuosilicate.

6. A bath in accordance with claim 1 wherein said salt is a neodymiumsalt.

7. A bath in accordance with claim 1 wherein said salt is neodymiumfluosilicate.

8. A bath in accordance with claim 1 wherein said salt is a lanthanumsalt.

9. A bath in accordance with claim 1 wherein said salt is predominantlylanthanum fiuosilicate.

10. A bath in accordance with claim 1 additionally containing asaturated fluorocarbon sulfonic acid.

11. A bath in accordance with claim 1 additionally containing asaturated fluorocarbon phosphonic acid.

12. A method of electrodepositing chromium which comprises electrolyzingan aqueous acidic hexavalent chromium solution containing about 100 to500 grams per liter of chromic acid, sulfate ion in a concentrationsufficient to have a chromic acid to sulfate ion ratio of from about to1 to about 300 to 1, and saturation concentrations of afluorine-containing salt of a rare earth metal selected from the groupconsisting of neodymium, prasedymium, lanthanum, gadolinium, samarium,yttrium and mixtures of said salts.

13. A method in accordance with claim 12 wherein said bath containssaturation concentrations of strontium sulfate.

14. A method in accordance with claim 12 wherein said salt is a didymiumsalt.

15. A method in accordance with said salt is didymium trifluoride.

16. A method in accordance with said salt is didymium fluosilicate.

17. A method in accordance with said salt is a neodymium salt.

18. A method in accordance with said salt is neodymium fluosilicatc.

19. A method in accordance with said salt is a lanthanum salt.

20. A method in accordance with claim 12 wherein said salt ispredominantly lanthanum fiuosilicate.

21. A method in accordance with claim 12 additionally containing asaturated fluorocarbon sulfonic acid.

22. A method in accordance with claim 12 additionally containing asaturated fluorocarbon phosphonic acid.

claim 12 wherein claim 12 wherein claim 12 wherein claim 12 whereinclaim 12 wherein References Cited UNITED STATES PATENTS 1,864,014 6/1932Ewing 20451 1,881,885 10/1932 Noble et al 204-51 2,640,022 5/1953Stareck 2045l 2,787,588 4/1957 Stareck et al 204-51 2,950,234 8/1960Johnson et a1. 204-5l FOREIGN PATENTS 882,936 7/1953 Germany.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. A BATH FOR THE ELECTRODEPOSITION OF CHROMIUM PLATE COMPRISING ABOUT100 TO 500 GRAMS PER LITER OF CHROMIC ACID, SULFATE ION IN ACONCENTRATION SUFFICIENT TO HAVE A CHROMIC ACID TO SULFATE RATIO OF FROMABOUT 75 TO 1 TO 300 TO 1, AND SATURATION CONCENTRATION OFFLUORINECONTAINING SALT OF A RARE EARTH METAL SELECTED FROM THE GROUPCONSISTING OF NEODYMIUM, PRASEODYMIUM, LANTHANUM, GADOLINIUM, SAMARIUM,YTTRIUM, AND MIXTURES OF SAID SALTS.